Wrong and Wrong!!! Why does a left sided attacking player need to possess defensive abilities? You are really buying into the BS Ranieri spews from his mouth. Does Marchionni help out the defense? He is so anemic, and doesn't even perform his function, which is presumably aiding the attack. Why does our LM need to support the defense then? Having 6 players (4 defenders, and 2 DMs) and the goalkeeper assisting the defense is sufficient cover I believe.
As regards his stature and size, well Messi and Silva are two diminutive and frail individuals, yet they wreak havoc on the opposition, so that renders your point about his size meaningless. He has trickery and pace, and his low center of gravity enables him to escape his markers, and cause problems for opponents. You don't have to be of a physical nature to succeed in either the role of AM, or LM.